Electrical power can be generated by many different methods, the most common involving the use of fossil or nuclear based fuels. As is known, these generating mechanisms have undesirable side effects, such as the production of toxic pollutants or reliance on dwindling natural resources. An alternative method of creating electrical power is to harness a renewable natural resource such as the wind.
A common type of wind turbine is a horizontal axis wind turbine, which typically uses two or three blades coupled together on a hub that rotates in response to a lifting force created by the wind. As used herein, the term “wind rotor” denotes the assembly that comprises a blade hub and a plurality of blades (airfoils). Generally, the wind rotor converts wind energy into the rotational energy that drives a generator. The hub is connected to a shaft that is connected to the generator, which supplies power to a load (e.g., electrical grid, residence, etc.). For certain wind turbines, a gearbox converts the rotation of the blades into a speed usable by the generator to produce electricity at a frequency that is proper for the load.
Although gear-driven wind turbines are still being made and used, direct-drive wind turbines are becoming more prevalent largely due to advances in systems for controlling this type of wind turbine. As its name implies, direct-drive wind turbines do not include a gearbox, but rather have a direct mechanical coupling between the wind rotor and generator so that the wind drives the wind rotor and the rotor within the generator together as a unit. Direct-drive wind turbines have an important advantage in lessened complexity, which typically results in direct-drive wind turbines being more reliable, and having longer life spans and lower costs of operation than their gear-driven counterparts.
Current wind turbine designs are focused on developing maximum power, which typically means designing a wind turbine that makes peak power at relatively high wind speeds (e.g., 24 and over miles per hour (mph) or 80-100% more than the average wind speed), using a high rpm generator. The need for high wind speeds means that these wind turbines are sited on ridgelines or other consistently windy areas, which are not desirable in many communities or even available as the vast majority of the population of the world lives in areas below 11 mph (5 m/s). In operation, the blades of these wind turbines have tip speeds that induce significant noise and result in animal (e.g., bird and bat) collisions. And lastly, the coefficient of power (Cp) (also referred to as the capacity factor) at less than peak power wind speed for wind turbines is poor because the design of the wind turbine causes it to operate inefficiently at less than the peak power wind speed. This design methodology also results in an additional cost to electrical systems because of the need for higher capacity transmission lines to accommodate the higher voltage that these turbines intermittently produce.